ArmVirtPkg/ArmPlatformLibQemu: Make IdMap.S BTI compatible

The IdMap.S asm source file has not executable content, but its lack of
a BTI annotation prevents the linker from marking any executables it
emits as BTI compatible if this object is part of the build. So add the
BTI note by hand.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>

ArmVirtPkg/Library/ArmPlatformLibQemu/IdMap.S

@@ -57,3 +57,5 @@ idmap:      /* level 0 */
   .quad     PAGE_XIP | (idx << 12)        // 2044 KiB of R-X flash mappings
   .set      idx, idx + 1
   .endr
+
+AARCH64_BTI_NOTE()